This invention is in the field of communication circuits. Embodiments are more specifically directed to the calibration of transmitter circuits operating at high frequencies, such as radio frequency (RF).
Solid-state integrated circuits are now commonly used in the transmission and reception of signals in a wide range of communications systems, both over physical media (e.g., wire lines and optical fibers) and wirelessly at radio frequencies (RF). In either case, but particularly in the wireless context, the measurement and control of the power of the transmitted signal is of significant importance for maximizing battery longevity. For the example of wireless RF communications, constraints on transmission power arise from the need of multiple communications links to co-exist both spatially and in the frequency domain. Measurement and control of signal power from transmitters of all types is also necessary to minimize electromagnetic noise that can undesirably affect unrelated electronic systems.
As fundamental in the art, variations in manufacturing process parameters in the fabrication of modern integrated circuits cause corresponding variations in electrical parameters such as transistor gain, circuit switching speed, and the like. In addition, integrated circuits are also typically sensitive to variations in power supply voltage and operating temperature. For integrated circuits implemented in communications transmitters, these variations are reflected in the transmitted signal power relative to the nominal levels for which the circuits are designed. Accordingly, calibration and control of the transmitted signal power is generally necessary in order for communications systems to meet system and regulatory requirements.
One conventional approach to measurement and control of power in transmitter integrated circuits is known in the art as loopback calibration. Loopback calibration refers to the use of circuitry on the transmitter integrated circuit itself to obtain an output power measurement used in the control of the transmitter power amplifiers. In conventional transmitters, however, this loopback approach provides only a relative measurement of transmitter power because the on-chip loopback receiver circuitry is subject to the same process, voltage, and temperature variations as is the transmitter circuitry. As such, an absolute measurement of output power, with precise reference to an external standard, is necessary to properly calibrate the transmitter function, necessitating the use of external test equipment.
Accordingly, calibration of the transmitter power amplifier for RF transmitters and transmitter/receivers (transceivers) necessitates the use of RF test circuitry. As known in the art, however, RF and other high frequency test equipment is very expensive, resulting in high test cost for conventional RF transmitting circuits. The high cost of RF test equipment also discourages the addition of incremental test capacity, which can result in test becoming a bottleneck in the manufacturing flow.